The rapid increase in sales of disposable nonwoven products over the past several years has intensified interest in improving emulsion polymers used to bind nonwoven fibers. Most conventional binders include a small amount of self-crosslinking agent, typically N-methylolacrylamide. The development of such self-crosslinking binders at the end of the 1950s was perhaps the most important factor in the growth and commercial acceptance of articles made from nonwoven staple fibers.
Unfortunately, nonwoven products made with such nonwoven binder compositions exhibit unacceptable loss in strength in the presence of water and other solvents. In addition, conventional binders containing phosphate surfactants exhibit poor adhesion to substrate including glass, metal and synthetics such as mylar. These shortcomings have been reduced in recent years by the use of adhesion promoting crosslinking comonomers and/or post-added crosslinkers.
Aminoplast chemistry is one of the most successful of the many chemistries employed in preparing nonwoven binder compositions. Particularly useful examples of compounds containing aminoplast functionality are N-methylolacrylamide (NMA) and urea-formaldehyde condensates. While these compounds are low in cost, compatible with aqueous emulsions, rapidly cured under acid catalysis and substrate reactive, they suffer from a major deficiency; the emission of low levels of formaldehyde, a suspected carcinogen. Many attempts have been made to overcome or minimize this deficiency, especially after the potential carcinogenicity and irritant properties of formaldehyde became widely recognized.
To reduce the level of formaldehyde in emulsion products, the use of O-alkylated NMA's such as isobutoxymethacrylamide (IBMA) or the use of 1:1 molar ratios of NMA with acrylamide were introduced. These materials did not, however, eliminate the presence of formaldehyde.
In recent years, investigation has focused on binder compositions incorporating carboxylate functionality in order to overcome the previously discussed deficiencies. The incorporation of acrylic acid and other carboxylic acid containing monomers into interpolymers is well known.
Crosslinking with metal ions including aluminum and zirconium has been disclosed as being useful for the insolubilization of carboxylic acid group-containing materials such as polyacrylic acid and starches containing carboxylic acid groups. Crosslinking afforded by such metal ions has been proposed to improve the mechanical properties of articles impregnated with nonwoven binders. U.S. Pat. Nos. 2,758,102 and 3,137,588 are illustrative.
U.S. Pat. No. 4,084,033 discloses a method for making nonwovens wherein an aqueous binder comprises a colloidal resin possessing a hydroxy-containing ligand. These resins are obtained by copolymerizing from about 92 wt % to about 99 wt % of a monomer or mixture of monomers including vinyl acetate and ethylene. A small amount of from about 0.1 wt % to about 3 wt % of a coordination metal complex is then added to the resin. Suitable central metallic atoms for such metal complexes include zirconium, chromium, nickel cobalt, cadmium, zinc, vanadium, titanium, copper and aluminum. An example of a suitable coordination compound includes zirconium ammonium carbonate.
U.S. Pat. No. 4,289,676 discloses copolymeric binder compositions containing from 3 to 6 wt % acrylamidoglycolic acid (AGA), up to 3 wt % N-methylolacrylamide and not less than 85 wt % of:
(a) a mixture of from 40-60 parts by weight of styrene and/or acrylonitrile and from 60-40 parts by weight of butadiene or PA0 (b) vinyl monomers selected from the group consisting of esters of acrylic acid or methacrylic acid with alkanols of 1 to 8 carbon atoms, vinyl esters and vinyl chloride, together with up to 40% by weight, based on total monomers (b), of acrylonitrile, styrene or butadiene,
and from 0 to 5% by weight of alpha, beta-monoolefinically unsaturated monocarboxylic acids and/or dicarboxylic acids of 3 to 5 carbon atoms and/or their amides, the said monomers being present as copolymerized units.
U.S. Pat. No. 4,447,570 teaches a binder composition for nonwoven fabrics. The binder comprises a base salt of a phosphate ester surfactant or carboxylate surfactant, a latex comprising vinyl acetate, ethylene and an olefinically unsaturated carboxylic acid interpolymer colloidally suspended in water. Additionally added is a polyvalent metal complex comprising a polyvalent metal ion (i.e. zirconium, aluminum, etc.) and counter ions or ligands which hinder interaction of the polyvalent metal ion with the carboxylate and phosphate groups of the surfactant at room temperatures. Heating serves to cure the binder by forming a crosslinked interpolymer caused by expelling or removing the counter ions or ligands and replacing them by the anionic groups of the surfactant and interpolymer.
U.S. Pat. No. 4,522,973 teaches a low temperature crosslinkable polymer emulsion containing methyl acrylamidoglycolate methyl ether (MAGME) and a crosslinking agent having a plurality of functional groups each capable at low temperature of replacing the alkoxy moiety of MAGME by nucleophilic substitution.